Filters for high performance cyclone vacuum cleaners, etc. are required to have high collection efficiency. Generally, for such filters, filter media including porous PTFE membranes, filter media obtained by adding binders to glass fibers and forming the mixtures into paper sheets (glass filter media), and filter media obtained by converting meltblown nonwoven fabrics into electret materials (electret filter media) are used. Filter media including porous PTFE membranes, among them, rarely suffer from problems such as the production of fibrils and self-dusting and have a smaller increase in the pressure drop during the use thereof. Because of these advantages, these filter media are expected to be used increasingly.
In a filter through which a large amount of air passes, such as a filter for a vacuum cleaner, the filter medium itself is required to have a certain degree of stiffness to prevent the filter from being deformed by the passing air. In a filter including a porous PTFE membrane, the porous PTFE membrane can be integrated with a gas-permeable supporting member for supporting the porous membrane to increase the stiffness of the filter medium to a level high enough to ensure sufficient stiffness. A wide variety of materials having better gas-permeability than the porous PTFE membrane can be used for the gas-permeable supporting member. Generally, materials such as nonwoven fabrics and meshes are used.
In order to ensure the gas-permeability of the filter medium and increase the stiffness thereof, for example, a nonwoven fabric of a polyester typified by polyethylene terephthalate (PET) may be used as the gas-permeable supporting member. JP 2005-253711 A discloses that a porous PTFE membrane and a polyester nonwoven fabric that is a gas-permeable supporting member may be integrated to form a filter medium.
PTFE, however, has poor adhesion to polyester, particularly PET. Therefore, in order to integrate the porous PTFE membrane directly with the PET nonwoven fabric that is a gas-permeable supporting member, heat lamination needs to be carried out at high temperature and high pressure. This type of lamination may cause clogging of the nonwoven fabric due to such high heat, which results easily in a significant decrease in the gas-permeability of the filter medium. Generally, PTFE has poor adhesion not only to PET but also to materials other than PET. Therefore, the same problems occur even when a gas-permeable supporting member made of a material other than a PET nonwoven fabric is used.
As a solution to these problems, a method can be employed in which a polyethylene (PE) nonwoven fabric having relatively good adhesion to PTFE is disposed as an adhesive layer between the porous PTFE membrane and the PET nonwoven fabric and these layers are bonded to each other by heat lamination. In this method, the lamination can be carried out at a lower temperature than the direct heat lamination of the porous PTFE membrane and the PET nonwoven fabric. As a result, a decrease in the gas-permeability of the filter medium caused by the clogging thereof can be inhibited. However, the bonding strength between the PET nonwoven fabric, which is a gas-permeable supporting member, and the PE nonwoven fabric, which is an adhesive layer, is not necessarily high. This causes problems such as peeling of the adhesive layer from the gas-permeable supporting member.
The combined use of an adhesive or a hot-melt agent is a good solution to improve the bonding between the gas-permeable supporting member and the adhesive layer. The combined use thereof, however, causes problems such as a decrease in the gas-permeability caused by clogging, a decrease in the yield caused by an increase in the number of manufacturing processes, and an occurrence of outgassing of the adhesive during the heat lamination.
In order to impart liquid repellency, such as water repellency or oil repellency, to the filter medium, the gas-permeable supporting member commonly is subjected to liquid repellent treatment. This liquid repellent treatment further decreases the bonding strength between the porous PTFE membrane and the gas-permeable supporting member.